This invention relates generally to the field of digitally controlled marking devices, and in particular to continuous type marking devices adapted to deposit solvent free marking materials.
Many different types of digitally controlled printing are known and currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires a very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, which are capable of producing high quality color images at high-speed and low cost, using standard paper.
Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop on demand ink jet.
Continuous ink jet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell. U.S. Pat. No. 3,373,437, which issued to Sweet et al. in 1967, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet.
U.S. Pat. No. 3,416,153, which issued to Hertz et al. in 1966, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. U.S. Pat. No. 3,878,519, which issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, which issued to Hertz in 1982 discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a drop formation point located within the electric field having an electric potential gradient. Drop formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect drops.
Conventional ink jet printers are disadvantaged in several ways. For example, in order to achieve very high quality images having resolutions approaching 900 dots per inch while maintaining acceptable printing speeds, a large number of discharge devices located on a printhead need to be frequently actuated thereby producing an ink droplet While high frequency actuation reduces printhead reliability, it also limits the viscosity range of the ink used in these printers. Typically, the viscosity of the ink is lowered by adding solvents such as water, etc. The increased liquid content results in slower ink dry times after the ink has been deposited on the receiver which decreases overall productivity. Additionally, increased solvent content can also cause an increase in ink bleeding during drying which reduces image sharpness negatively affecting image resolution and other image quality metrics.
Conventional ink jet printers are also disadvantaged in that the discharge devices of the printheads can become partially blocked and/or completely blocked with ink. In order to reduce this problem, solvents, such as glycol, glycerol, etc., are added to the ink formulation, which can adversely affect image quality. Alternatively, discharge devices are cleaned at regular intervals in order to reduce this problem. This increases the complexity of the printer and educes effective printing time.
Another disadvantage of conventional ink jet printers is their inability to obtain true gray scale printing. Conventional ink jet printers produce gray scale by varying drop density while maintaining a constant drop size. However, the ability to vary drop size is desired in order to obtain true gray scale printing.
Other technologies that deposit a dye onto a receiver using gaseous propellants are known. For example, Peeters et al., in U.S. Pat. No. 6,116,718, issued Sep. 12, 2000, discloses a print head for use in a marking apparatus in which a propellant gas is passed through a channel, the marking material is introduced controllably into the propellant stream to form a ballistic aerosol for propelling non-colloidal, solid or semi-solid particulate or a liquid, toward a receiver with sufficient kinetic energy to fuse the marking material to the receiver. There is a problem with this technology in that the marking material and propellant stream are two different entities and the propellant is used to impart kinetic energy to the marking material. When the marking material is added into the propellant stream in the channel, a non-colloidal ballistic aerosol is formed prior to exiting the print head. This non-colloidal ballistic aerosol, which is a combination of the marking material and the propellant, is not thermodynamically stable/metastable. As such, the marking material is prone to settling in the propellant stream which, in turn, can cause marking material agglomeration, leading to nozzle obstruction and poor control over marking material deposition.
Technologies that use supercritical fluid solvents to create thin films are also known. For example, R. D. Smith in U.S. Pat. No. 4,734,227, issued Mar. 29, 1988, discloses a method of depositing solid films or creating fine powders through the dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution to create particles of the marking material in the form of fine powders or long thin fibers, which may be used to make films. There is a problem with this method in that the free-jet expansion of the supercritical fluid solution results in a non-collimated/defocused spray that cannot be used to create high resolution patterns on a receiver. Further, defocusing leads to losses of the marking material.
As such, there is a need for a technology that permits high speed, accurate, and precise delivery of marking materials to a receiver continuously to create high resolution images. There is also a need for a technology that permits continuous delivery of ultra-small (nano-scale) marking material particles of varying sizes to obtain gray scale. There is also a need for a technology that permits continuous delivery of solvent free marking materials to a receiver. There is also a need for a technology that permits high speed, accurate, and precise imaging on a receiver having reduced material agglomeration characteristics.
According to one feature of invention an apparatus for continuously delivering a solvent free marking material to a receiver includes a printhead with a discharge device. The discharge device has an outlet and is in fluid communication with a pressurized reservoir of a thermodynamically stable mixture of a compressed fluid solvent and a marking material. The marking material becomes free of the solvent after being ejected through the discharge device. A deflection mechanism is positioned relative to the outlet of the discharge device. The deflection mechanism is adapted to selectively deflect the marking material away from a first path to a second path.
A gutter can be positioned at an end of the first path which collects the solvent free marking material. A receiver transporting mechanism can be positioned at an end of the second path and is adapted to provide a receiver on which the solvent free marking material is deposited.
According to another feature of the invention a method of continuously delivering a solvent free marking material to a receiver includes providing a pressurized reservoir of a thermodynamically stable mixture of a compressed fluid solvent and a marking material. The mixture of the thermodynamically stable mixture of the compressed fluid solvent and the marking material is delivered along a first path toward a gutter or, alternatively, a receiver transport mechanism. The marking material becomes free of the solvent. The marking material is selectively deflected away from the first path to a second path to a receiver positioned on a receiver transport mechanism or, alternatively, a gutter.